Secret documents disclosing efforts by US, the UK and other NATO allies to build up Ukraine’s forces ahead of a new offensive against Russian troops appear to have been leaked online.

The US defence department is investigating who is responsible for the potentially damaging leak showing screenshots of classified military documents, which were posted on Twitter and Telegram.

The New York Times, which first reported the breach, quoted military analysts as saying the files appear to have been modified in certain parts, which could point to an attempt by Moscow to spread disinformation.

Wall Street Journal correspondent Yaroslav Trofimov said Russian propaganda channels appeared to have photoshopped at least one of the documents after the original ones were posted.

Ukraine war – latest: Russia makes ‘further gains’ in Bakhmut after stalled advance

Mr Trofimov noted how there was suddenly a significant increase in the number of Ukrainian casualties and equipment losses recorded and a massive decrease in the Russian battle damage.

Ukrainian presidential official, Mykhailo Podolyak, said the leak looked like a Russian disinformation operation, saying that it contained a “very large amount of fictitious information”.

The act of stealing secret documents and leaking them either with or without modifications is a long-standing weapon of information warfare designed to undermine an opponent.

It would benefit Russia for information about Ukraine’s battle plans and Western support to be leaked online.

The classified files – including one marked “top secret” and another marked “secret” – are dated from late February and early March.

They do not reveal specific dates or details about Ukraine’s anticipated spring offensive in the east and south of the country.

But they do offer clues about the kind of military formations that Western allies are helping their Ukrainian partners build up.

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Putin: US-Russia relations ‘in deep crisis’

Leaked plans widely shared

The New York Times said US officials were trying to have the files taken down off the social media sites.

However, as of Friday morning, versions of the leaks were still being widely shared.

It comes as Russian forces have made further gains in embattled Bakhmut and now Ukraine’s supply route is “severely threatened”, according to British intelligence.

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Bakhmut, a city in the east of Ukraine, has become the site of fierce fighting for several months, with both sides losing so many troops it has been described as a “meat grinder”.

Since late March, Russia’s advance had mostly stalled but now it has “likely advanced into the town centre and seized the west bank of the Bakhmuta River”, the UK’s Ministry of Defence (MoD) said in its latest Defence Intelligence update.

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As a result, Ukraine’s key supply route to the west of the town is under threat, which could hamper their fighting efforts.

“Russian regular forces, likely including airborne troops, have probably reinforced the area and Russia is again using artillery more effectively in the sector,” the MoD added.

One of the reasons for this “regained momentum” is that Wagner mercenary fighters and Russian commanders have “paused their ongoing feud” and started working together, the MoD said.

Meanwhile, the general staff of the Ukrainian armed forces said on Friday that more than 40 “enemy attacks” had been repelled in the past 24 hours.

Russia fired 53 rockets, five missiles and 18 air strikes on various parts of the country, according to the unit, which oversees operational management of Kyiv’s troops.

Moscow has continued to focus its efforts in Lyman, Bakhmut, Avdiivka and Marinka, all of which are in the eastern Donetsk region, it added.